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The aim of the present work was to study the protein utilization during energy undernutrition in sheep. In four different trials, the effects of varying levels of protein with submaintenance amounts of energy on the energy &'38 N balances of sheep were studied using the intragastric infusion technique and the respiration chamber. The effects of change in energy and protein supply on plasma metabolites and hormonal concentrations were also studied. When animals were given protein well in excess of their maintenance requirement with little or no non-protein energy, they attained positive N balance although they were in negative energy balance, apparently by efficient utilization of endogenous energy (presumably body fat). As body fat was used to fuel the energy needed for protein retention, fat and protein deposition were negatively correlated. However, at very high level of casein infusion, the oxidized component of the supplied protein can contribute up to 36&'37 of the total ME requirement. About 16 kJ of endogenous energy was used for each g of protein accretion. The efficiency of endogenous energy utilization ranged between 0.56 to 0.60. There was no clear evidence, that there is any minimum level of body fatness which is necessary before body fat can be utilized to support protein retention during exogeneous non-protein energy restriction. Protein utilization during exogenous energy restriction was found to be more affected by the growth potential than the adiposity of the animal. Both fasting-heat production &'38 N excretion were reduced when the glucogenic needs of animals were met. Similarly plasma glucose, -hydroxybutyrate and free fatty acids concentrations were not affected by the energy status of the animals, when glucose requirements of the animals were met.

Thesis (M. S.)--Oklahoma State University, 2005. / Vita. Includes bibliographical references (p.58-63).

Master of Science / Department of Animal Sciences and Industry / K C Olson / Our objective was to evaluate the effects of pre- and post-partum bolus injections of a trace mineral solution on beef cow reproductive performance, body weight (BW) change, and body condition score (BCS) change and on performance of suckling calves. Mature beef cows (n = 460; initial BW = 497 ± 89 kg, initial BCS = 5.4 ± 0.74) were stratified by BCS, parity, and predicted calving date and assigned randomly to 1 of 2 treatments: 1) subcutaneous trace mineral (TM) injection containing 15 mg/mL Cu, 5 mg/mL Se, 10 mg/mL Mn, and 60 mg/mL Zn or 2) subcutaneous injection of physiological saline (SA). Injections were administered to cows (1 mL / 90 kg BW) 105 days before the first projected calving date and again 30 days before fixed-time AI. Calves received the same treatment as their dams and were injected (1 mL / 45 kg BW) at birth and again at 71 ± 21 days of age. Cows grazed native pastures for the duration of the study; trace mineral supplements and white salt were available to all cattle ad libitum before and during the study. Ovulation was synchronized using a 5-d CO-Synch + CIDR protocol and cows were inseminated 60 to 64 hours after CIDR removal. Cows were exposed to fertile bulls for natural-service breeding 10 days after AI for 35 to 50 days. Conception to AI and final pregnancy rate were assessed 36 days after AI with ultrasound and 120 days after AI via rectal palpation. Change in BW and BCS from initiation of the study to calving and from AI to weaning did not differ (P ≥ 0.15) between TM and SA cows. Conversely, TM cows had greater (P = 0.04) BCS increase than SA cows between calving and AI. Calf BW at birth, ADG, and age-adjusted weaning BW did not differ (P ≥ 0.36) between treatments. Proportion of cows with estrus cycles 17 and 8 days before ovulation synchronization was similar (P ≥ 0.51) between treatments. Conception to AI was greater (P = 0.05) for cows receiving TM (60.2%) than for cows receiving SA (51.2%); however, overall pregnancy did not differ (P = 0.24) between treatments and averaged 92%. Under the conditions of our study, pre- and post-partum TM injections improved conception to fixed-time AI by beef cows.

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Master of Science / Department of Animal Sciences and Industry / Barry J. Bradford / Two experiments were conducted to explore potential cellular and biochemical mechanisms by which zinc may impact milk production and mammary health of dairy cattle. For the first experiment, twelve lactating Holstein cows were enrolled in a replicated 3 x 3 Latin square, and provided a balanced diet supplemented with either 30 ppm zinc as ZnSO₄ (30-ZS), 60 ppm zinc as ZnSO₄ (60-ZS), or 60 ppm zinc as a mixture of ZnSO₄ and Zn methionine (60-ZM). Blood and milk were collected and analyzed for markers of blood-milk leak and dairy food properties of milk. Milk cells were also isolated and analyzed for RNA abundance of genes related to zinc-mediated apoptosis. Aside from a tendency for 60-ZS to increase feed intake and 60-ZM to increase milk fat compared to 30-ZS, no effects were observed on performance, markers of blood-milk leak, transcript abundance of selected genes, or dairy food properties of milk. A cell culture study was also conducted using immortalized bovine mammary epithelial (MAC-T) cells. Messenger RNA abundance of GPR39—a G protein-coupled receptor shown to regulate cellular activity in response to extracellular zinc concentrations in other epithelial cell lines—was knocked down using RNA interference. Following GPR39 knockdown, cells were treated with 0 or 100 μM zinc. Phosphorylation of the kinases ERK1/2 and AKT was measured in GPR39-knockdown and control cells treated with either 0 or 100 μM zinc. No effect of zinc treatment or GPR39 knockdown was detected on kinase phosphorylation. Interestingly, the 100 μM zinc treatment showed a tendency to increase GPR39 mRNA abundance in control cells. There remain many questions about the cellular mechanisms whereby zinc can impact milk production in dairy cattle.

Zinc (Zn) has numerous functions and life sustaining processes depend on the presence of Zn within the body. Zinc sulfate is a common form of Zn supplemented in the beef industry. Inorganic sulfate based minerals have been associated with negative rumen effects, mainly, decreased rumen fermentation and protozoa numbers. Our studies concluded no difference in digestibility of three different forms of Zn (zinc glycinate, zinc sulfate, and zinc hydroxychloride). Although overall digestibility did not change, there were specific mineral concentration differences after steers consumed concentrate treatment for 10 days. These differences implied that the mineral fed was consumed and absorbed in post-treatment sample collection of rumen fluid, plasma, and liver. Overall liver Zn levels were within normal ranges and implies the zn fed was absorbed. Based on the results of this study bioavailability of, zinc glycinate, zinc sulfate, or zinc hydroxychloride were similar.